all: make unit.Converter concrete and rename to Metric

An interface for scaling dp and sp is overkill, at least for all
current uses. Make it a concrete struct type, and rename it to the
shorter and more precise Metric.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

layout/list.go

@@ -121,7 +121,7 @@ func (l *List) Dragging() bool {
 }
 
 func (l *List) update() {
-	d := l.scroll.Scroll(l.ctx, l.ctx, l.ctx.Now, gesture.Axis(l.Axis))
+	d := l.scroll.Scroll(l.ctx.Metric, l.ctx, l.ctx.Now, gesture.Axis(l.Axis))
 	l.scrollDelta = d
 	l.Position.Offset += d
 }